1. Field of the Invention
The invention relates to a beamforming technique, and more particularly to an OFDM beamformer, and a signal receiving system incorporating the same.
2. Description of the Related Art
The wireless communication system design has a current trend toward orthogonal frequency-division multiplexing (OFDM) for high-speed data transmission over frequency-selective fading channels. OFDM wireless communications with high data rates have been successfully applied to digital audio broadcasting (DAB), digital video broadcasting (DVB) and wireless local area networks (WLAN). When different users appear in the same spectrum, co-channel interference (CCI) may occur and signal communication quality may degrade. Thus, antenna array beamforming has been developed to reduce CCI.
In recent years, antenna array schemes have been proposed to mitigate CCI and improve bit error rate (BER) performance. For pre-fast Fourier transform (pre-FFT) beamforming, low computational complexity is required but performance degradation limits its applications. For post-FFT beamforming, time-domain signals received at a receiving end are transformed into frequency-domain signals prior to beamforming process. However, a beamforming weight vector for each subcarrier is obtained through a large number of computational operations.
A conventional post-FFT beamforming scheme has been proposed in an article by Y. Liu and Y. Yang, entitled “Efficient Adaptive Array Receiver for OFDM based Wireless Local Area Networks (WLAN),” IEEE Trans, Consumer Electronics, vol. 50, no. 4, pp. 1101-1106, November 2004. In the conventional post-FFT beamforming scheme, one set of weight vectors are computed from a specific subcarrier and are utilized to process all the subcarriers, and channel estimation and equalization are employed to compensate channel variations and achieve comparable performance.
A conventional blind beamforming scheme has been proposed in an article by Y. F. Chen and C. S. Wang, entitled “Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems with Virtual Carriers,” IEEE Trans. Veh. Technol., vol. 56, no. 4, pp. 1837-1844, July 2007. In the conventional blind beamforming scheme, a single weight vector is applied to all subcarriers in an OFDM-based system. For an OFDM receiver, beamformer processing is arranged after parallel-to-serial converting. Thus, a weight vector of the beamformer can be computed without a training sequence. In addition, channel impulse response estimation can be omitted.
It is noted that bit error rate (BER) performance is not discussed in the conventional post-FFT beamforming scheme and the conventional blind beamforming scheme. Therefore, improvements may be made to the above techniques.